Polymer/carbon-nanotube interpenetrating networks and process for making same

ABSTRACT

The present invention is directed to new methods for combining, processing, and modifying existing materials, resulting in novel products with enhanced mechanical, electrical and electronic properties. The present invention provides for polymer/carbon nanotube composites with increased strength and toughness; beneficial for lighter and/or stronger structural components for terrestrial and aerospace applications, electrically and thermally conductive polymer composites, and electrostatic dissipative materials. Such composites rely on a molecular interpenetration between entangled single-wall carbon nanotubes (SWNTs) and cross-linked polymers to a degree not possible with previous processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/632,196, filed on Jan. 17, 2007, which is a national phaseapplication of PCT/US2005/026284, filed Jul. 22, 2005, which claimspriority to U.S. Provisional Patent Application No. 60/590,263, filed onJul. 22, 2004. The entirety of the above-referenced applications areincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under the Office ofNaval Research Grant No. N00014-03-1-0296, awarded by the United StatesNavy; and Grant No. NCC-1-02038, awarded by the National Aeronautics andSpace Administration. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs), comprising multiple concentric shells andtermed multi-walled carbon nanotubes (MWNTs), were discovered by Iijimain 1991 (Iijima, Nature, 1991, 354, 56). Subsequent to this discovery,single-walled carbon nanotubes (SWNTs), comprising a single graphenerolled up on itself, were synthesized in an arc-discharge process usingcarbon electrodes doped with transition metals [Iijima et al., Nature,1993, 363, 603; and Bethune et al., Nature, 1993, 363, 605).

The seamless graphitic structure of single-walled carbon nanotubes(SWNTs) endows these materials with exceptional mechanical properties:Young's modulus in the low TPa range and (estimated) tensile strengthsin excess of 37 GPa (Treacy et al., Nature 1996, 381, 678; Ruoff et al.,Carbon 1995, 33, 925; Yakobson et al., Phys. Rev. Lett. 1996, 76, 2411;Lourie et al., J. Mater. Res. 1998, 13, 2418; Iijima et al., J. Chem.Phys. 1996, 104, 2089; Cornwell et al., Solid State Comm. 1997, 101,555; Lu, Phys. Rev. Lett. 1997, 79, 1297; Saito et al. PhysicalProperties of Carbon Nanotubes, Imperial College Press: London (1998);Yu et al., Phys. Rev. Lett. 2000, 84, 5552). Electron microscopy studiesof SWNTs have shown that the nanotubes, although extremely strong intension, are very flexible in bending (Lourie et al. Phys. Rev. Lett.1998, 81, 138; Vigolo et al., Science 2000, 290, 1331). Consequently,one would expect that incorporation of SWNTs as reinforcement inpolymeric matrices could generate composites with greatly enhancedstrength and toughness. To achieve this goal, the composites mustpossess sufficient structural continuity, so that external loads,imposed on the composite can be efficiently shared by the soft polymermatrix and the ultra high-strength nanotubes.

Several investigators have prepared a variety of composites, byembedding SWNTs in epoxy resins and other polymer matrices (Lozano etal., J. Appl. Polym. Sci. 2001, 79, 125; Lozano et al., J. Appl. Polym.Sci. 2001, 80, 112; Schadler et al., Appl. Phys. Lett. 1998, 73, 3842;Ajayan et al., Adv. Mater. 2000, 12, 750). In most cases, the resultingcomposites have shown unremarkable mechanical properties and poorpolymer-nanotube adhesion. The composites fractured at stresses,comparable to those of the non-reinforced polymer, with intact nanotubespulling out from the matrix of either fragment. In all of thesepreparations the nanotubes were present in the matrix as discreteentities or small bundles. Hence, structural continuity within thecomposite depended entirely on adhesive (secondary) bonds betweenindividual nanotubes and polymer chains. Given the marked difference ininterfacial free energy between carbon nanotubes and organicmacromolecules, it is not surprising that the adhesive bonds betweenthese two entities are poor and the presence of discrete nanotubes doesnot strengthen the composite.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to new methods for combining,processing, and modifying existing materials, resulting in novelproducts with enhanced mechanical, electrical and electronic properties.The present invention provides for polymer/carbon nanotube (polymer/CNT)composites with increased strength and toughness; typically beingbeneficial for lighter and/or stronger structural components for avariety of terrestrial and aerospace applications, electrically andthermally conductive polymer composites, and electrostatic dissipativematerials. Such composites rely on a molecular interpenetration betweenentangled single-walled carbon nanotubes (SWNTs) and cross-linkedpolymers to a degree not possible with previous processes. As CNTs, andespecially SWNTs, can be viewed as polymers themselves, such polymer/CNTcomposites can be viewed as hybrid polymer systems wherein the nanotubecomponent provides reinforcement.

In some embodiments, the present invention is directed to methods ofproducing polymer/CNT composites, the methods comprising the steps of:(1) providing entangled agglomerates of CNTs; (2) processing theagglomerates such that they are penetrated with polymer or polymerprecursor material; (3) optionally linking the agglomerates; and (4)optionally permitting bonding between the polymer material and theagglomerates and/or between the agglomerates.

In some embodiments, the present invention is directed at methods ofproducing polymer/CNT composites, the methods comprising the steps of:(1) introducing CNTs and prepolymer molecules into a solvent to form asolvent mixture; (2) atomizing the solvent mixture into micro-dropletsvia spraying; (3) rapidly removing the solvent from the micro-dropletsand, simultaneously, fully or at least partially curing the prepolymerto provide solid polymer/CNT particles; and (4) depositing the solidpolymer/CNT particles on a surface to form a polymer/CNT compositelayer.

In some embodiments, a B-stage powder (partially cured prepolymer) isformed, wherein said powder is comprised of particles, the particlescomprising: (1) CNTs; (2) polymer material that forms aninterpenetrating network with the CNTs and which is partially cured atleast to an extent so as to prevent to re-bundling of the CNTs; and (3)at least one unactivated curing agent capable of further curing thepolymer material.

In some embodiments, a plurality of B-stage particles comprisingpartially cured polymer material can be further cured to form bulkobjects comprising interpenetrating networks of CNTs and polymericmaterial. In some of these embodiments, there is an incipient-wetting ofthe B-stage particles with additional CNT material. In some or other ofthese embodiments, additional polymeric material may be added to theB-stage particles. In some embodiments, the further curing is doneduring a high-shear extrusion process. Additionally or alternatively,the further curing can be done in conjunction with solid free-formfabrication. One such exemplary solid free-form fabrication process israpid prototyping.

In some embodiments, the interpenetrating networks of carbon nanotubesand polymer are used to reinforce other fiber forms, such as glassfibers, Kevlar®, carbon fibers, or other fiber systems.

The foregoing has outlined rather broadly the features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of the formation of polymer/CNTinterpenetrating networks, wherein in the initial mixing step (1)tangled CNTs are dispersed into the liquid monomer system, after theinterpenetration step (2) monomer molecules infuse/penetrate into thetangled CNTs networks causing them to swell, and after the curing step(3) macromolecular networks are generated by interpenetration of polymerinto the expanded CNTs networks, forming a high-modulus, high-strengthcomposite, wherein no phase separation occurs;

FIGS. 2(A)-2(B) depict SEM micrographs of as-received SWNTs;magnification of (A) 25,000 times, (B) 50,000 times, (C) 100,000 times;

FIG. 3 is a schematic diagram of a spray process that generatescontinuous buildup of a polymer/CNT composite, wherein the thickness ofthe composite increases with the duration of spray, in accordance withsome embodiments of the present invention;

FIG. 4 is an illustrative cross-sectional view of an apparatus forspraying SWNT/prepolymer solutions, in accordance with some embodimentsof the present invention;

FIG. 5 is a schematic diagram showing a system for simultaneouslysonicating and spraying a dispersion/solution of CNT/polymer/organicliquid, in accordance with some embodiments of the present invention;

FIGS. 6( a)-6(d) illustrate the formation of a polymer/CNT layercomprising a CNT concentration gradient in the direction of the layerthickness, in accordance with some embodiments of the present invention,wherein such a gradient is formed by changing the concentration of theCNTs in the CNT/polymer/organic liquid during processing;

FIGS. 7( a)-7(f) illustrate the formation of a polymer/CNT layercomprising alternating sub-layers of different CNT concentration, inaccordance with some embodiments of the present invention, wherein sucha layered structure is formed by alternating the deposition ofCNT/polymer/organic liquid with shots comprising differentconcentrations of CNTs (or a complete lack of CNTs);

FIG. 8 is a schematic diagram showing simultaneous solvent evaporationand initial polymerization as the droplets travel from a spray nozzle toa deposition surface (a-b), and illustrating how the B-stageCNT/prepolymer materials can be introduced into downstream processessuch as compression (d) and extrusion (e), in accordance with someembodiments of the present invention;

FIG. 9 depicts SEM micrographs of the fracture surface of a 1 weight %as-received SWNT/epoxy sprayed composite;

FIGS. 10(A) and 10(B) depict SEM micrographs of the fracture surface ofa 0.1 weight % as-received SWNTs/epoxy sprayed composite: (A) 5,000times (B) 50,000 times;

FIGS. 11(A) and 11(B) depict SEM micrographs of the fracture surface of0.1 weight % carboxylic acid end-functionalized SWNT/epoxy sprayedcomposite: (A) 5,000 times (B) 50,000 times;

FIGS. 12(A) and 12(B) depict SEM micrographs of the fracture surface of0.1 weight % carboxylic acid sidewall-functionalized SWNT/epoxy sprayedcomposite: (A) 5,000 times (B) 50,000 times;

FIG. 13 depicts Raman spectra of the 0.1 weight % as-received SWNT/epoxysprayed composite after exposure to a polarized laser beam, changing theincident angle: (a) 0 degrees, (c) 45 degrees, and (b) 90 degrees (RBMrepresents radial breathing mode and G represents tangential mode);

FIG. 14 summarizes the relative Raman intensity ratios of the RBM/Gpeaks depicted in FIG. 13;

FIGS. 15( a) and 15(b) are optical microscopy images depicting alignedSWNT/epoxy on a substrate after a single shot (spay) onto the preheatedsubstrate, wherein the associated schematic diagrams show the differentviews obtained by changing the focal point on the same sample during theimaging process;

FIGS. 16( a)-16(i) illustrate a possible growth mechanism of alignedpalm tree-like SWNT/epoxy columns, in accordance with some embodimentsof the present invention; and

FIG. 17 is a schematic diagram of spun fiber comprising aligned SWNTropes interconnected with an oriented macromolecular network, inaccordance with some embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to methods of preparing polymer/carbonnanotube (polymer/CNT) composites wherein such methods promote molecularinterpenetration between entangled carbon nanotubes (CNTs) andcross-linked polymers. The present invention is also directed to thepolymer/CNT composites produced by such methods. While the making and/orusing of various embodiments of the present invention are discussedbelow, it should be appreciated that the present invention provides manyapplicable inventive concepts that may be embodied in a variety ofspecific contexts. The specific embodiments discussed herein are merelyillustrative of specific ways to make and/or use the invention and arenot intended to delimit the scope of the invention.

Carbon nanotubes (CNTs), according to the present invention, include,but are not limited to, single-walled carbon nanotubes (SWNTs),multi-walled carbon nanotubes (MWNTs), double-walled carbon nanotubes(DWNTs), buckytubes, fullerene tubes, tubular fullerenes, graphitefibrils, and combinations thereof. Such carbon nanotubes can be made byany known technique including, but not limited to the HiPco® process,(Bronikowski et al., J. Vac. Sci. Technol. A. 2001, 19, 1800), arcdischarge (Ebbesen, Annu. Rev. Mater. Sci. 1994, 24, 235), laser oven(Thess et al., Science 1996, 273, 483), flame synthesis (Vander Wal etal., Chem. Phys. Lett. 2001, 349, 178), chemical vapor deposition (U.S.Pat. No. 5,374,415), wherein a supported (Hafner et al., Chem. Phys.Lett. 1998, 296, 195) or an unsupported (Cheng et al., Chem. Phys. Lett.1998, 289, 602; and Nikolaev et al., Chem. Phys. Lett. 1999, 313, 91)metal catalyst may also be used, and combinations thereof. Depending onthe embodiment, the CNTs can be subjected to one or more processingsteps prior to subjecting them to any of the processes of the presentinvention. In some embodiments, the CNTs are separated based on aproperty selected from the group consisting of chirality, electricalconductivity, thermal conductivity, diameter, length, number of walls,and combinations thereof. See O'Connell et al., Science 2002, 297, 593;Bachilo et al., Science 2002, 298, 2361; Strano et al., Science 2003,301, 1519. In some embodiments, the CNTs have been purified. Exemplarypurification techniques include, but are not limited to, those by Chianget al. (Chiang et al., J. Phys. Chem. B 2001, 105, 1157; Chiang et al.,J. Phys. Chem. B 2001, 105, 8297). In some embodiments, the CNTs havebeen cut by a cutting process. See Liu et al., Science 1998, 280, 1253;Gu et al., Nano Lett. 2002, 2(9), 1009. The terms “CNT” and “nanotube”are used synonymously herein. Furthermore, while much of the discussionherein involves SWNTs, it should be understood that many of the methodsand/or compositions of the present invention utilizing and/or comprisingSWNTs can also utilize and/or comprise MWNTs or any of the other typesof CNTs discussed above.

Generally, polymer/CNT composites of the present invention compriseinterpenetrating nanofiber networks, the networks comprising mutuallyentangled carbon nanotubes intertwined with macromolecules in across-linked polymer matrix. Key to the successful practice of thepresent invention is the infusion of organic molecules capable ofpenetrating into the clumps of tangled CNTs, thereby causing thenanotube networks to expand and resulting in exfoliation. Subsequent insitu polymerization and curing of the organic molecules generatesinterpenetrating networks of entangled CNTs or CNT nanofibers (ropes),intertwined with cross-linked macromolecules. This is shownschematically in FIG. 1, wherein in the initial mixing step (1) tangledCNTs are dispersed into the liquid monomer system, after theinterpenetration step (2) monomer molecules infuse/penetrate into thetangled CNT networks causing them to swell, and after the curing step(3) macromolecular networks are generated by interpenetration of polymerinto the expanded CNT networks, forming a high-modulus, high-strengthcomposite, wherein no phase separation occurs;

It should be noted that in their nascent state, SWNTs exist in the formof tangled networks, as exemplified by scanning electron micrographs,shown in FIGS. 2(A) and 2(B) taken at successively highermagnifications. These micrographs also show impurities, such as carbonblack, soot, catalyst particles, etc., which are usually present. Insome embodiments nascent, unpurified (as-produced) SWNTs are used—whichmay provide additional reinforcement (or other desired properties) tothe composite by virtue of their impurities. Furthermore, theseembodiments eliminate the usual purification and processing steps thatadd significantly to the cost, and possibly degrade the strength, of theSWNT assemblies.

Generally, the difference in surface free energy between CNTs andpolymerizable organic molecules (prepolymers or monomers) issufficiently high, so that when the two components are mixed, theyremain separated into distinct phases: clumped carbon nanotubessuspended in the otherwise continuous organic phase. Use of mechanicalenergy, such as sonication or high shear, may, at best, reduce the sizeof the individual nanotube clumps, but it does not enable substantialpenetration of the organic molecules.

A very small number of organic liquids (solvents), such asN,N-dimethylformamide (DMF), are capable of dissolving carbon nanotubesat low concentrations (Ausman et al., J. Phys. Chem. B 2000, 104, 8911),although such solutions may not always be solutions in the truethermodynamic sense. The same liquids are also solvents for a widevariety of pre-polymers. In some embodiments, the present inventiontakes advantage of this mutual solubility and combines it withatomization and spraying techniques to generate composites with thedesired interpenetrating network structures. Generally, the methodsinvolved in fabricating CNT-polymer interpenetrating networks comprisethe following steps: (1) introducing prepolymer molecules and CNTs intoone of the few organic solvents that can disperse or dissolve smallamounts of CNTs in order to form a solvent mixture; (2) promoting CNTdissolution within the mixture using a technique selected from the groupconsisting of sonication, heating, mechanical shear, combinationsthereof, and/or other appropriate means; (3) atomizing the (possiblyheated) solution into fine droplets, using standard spraying equipment;and (4) depositing the droplets onto a surface via spraying, using heatand, possibly, vacuum to effect rapid and simultaneous evaporation ofthe solvent and initial curing of the prepolymer, such that the dropletssolidify before the CNTs have a chance to separate from the polymerizingsystem and re-aggregate due to solvent depletion.

In some embodiments, to generate composites in a desired shape and/orlevel of homogeneity, the above-described spray is deposited onto amoving surface, such as a rotating disc, in order to continuously builda homogeneous layer of composite material with a desired thickness, asshown schematically, and in exemplary terms, in FIG. 3 and FIG. 4.Referring to FIG. 3, a spray nozzle 302 deposits a polymer/CNT layer 305on a rotating disk 301, the rotating disk being heated by a hot air gun304 and the deposited layer being heated by an infrared lamp 303. FIG. 4illustrates how the parts of FIG. 3 can be integrated into an apparatus400 that is partially enclosed in a cabinet 403, wherein the rotatingdisk 301 is driven by a variable speed motor 401 and a gear and chaindrive 402.

Test specimens and other objects can be cut from the above-describedpolymer/CNT layer. The process parameters can be easily controlled byadjusting the infrared lamp temperature, the temperature of the hot airgun, the air flow rate, the rotating stage speed, the spray pressure,the spray angle, etc. Multiple spray guns can be attached onto theabove-described system to spray multiple distinct materials, such as aspray gun 1 spraying prepolymer/organic liquid A and a spray gun 2spraying CNTs/organic liquid B. Spray guns spraying multipleconcentrations of CNTs/prepolymer/organic liquid can also be installedin this system.

Alternatively, in some embodiments, to enhance the homogeneity of theCNTs/prepolymer, the mixture of CNTs/prepolymer/organic liquid can bemechanically dispersed just before spraying through the nozzle by abuilt-in probe-type sonicator, which is just above the mixturereservoir, as shown schematically in FIG. 5. Referring to FIG. 5, aprobe-type sonicator 501 sonicates a dispersion/solution ofCNT/prepolymer/organic liquid immediately before being sprayed throughnozzle 502 and onto preheated surface 503.

In some embodiments, the concentration of CNTs in the composite can becontinuously changed by adjusting the spraying of the solution ofCNTs/prepolymer/organic liquid. Referring to FIGS. 6( a)-6(d), theinitial step of spraying only prepolymer/organic liquid is depicted inFIG. 6( a), and the gradual increasing of CNT concentration in theCNTs/prepolymer/organic liquid is depicted in FIGS. 6( b)-6(d). Thefinal product can yield hybrid properties, if, e.g., one side is aconducting composite and the other side is an insulator.

In some embodiments, the concentration of CNTs in the composite can bemodulated by alternatively establishing sub-layers of different CNTconcentration by alternating the spraying of CNTs/prepolymer/organicliquid solutions comprising different concentrations of CNTs. Referringto FIGS. 7( a)-7(f), the initial step of spraying onlyprepolymer/organic liquid (0% CNTs) is depicted in FIG. 7( a), the nextstep of spraying a CNTs/prepolymer/organic liquid solution of alternateCNT concentration (e.g., 20% CNTs) is depicted in FIG. 7( b), and FIGS.7( c)-7(e) depict the repetition of the steps depicted in FIGS. 7( a)and 7(b) until reaching a desired thickness, and FIG. 7( f) depicts theexpected final sample.

Alternatively, in some embodiments, a prepolymer system is employed withtwo independent curing agents. In such embodiments, the first agent,activated during the spraying and deposition of the droplets, providesan extent of polymerization sufficient to quickly solidify the dropletsto a “B-stage” (partially cured prepolymer), thus preventing SWNTreaggregation. This step is shown schematically in FIG. 8, wherein theB-stage solid particles or aggregates are formed en route to a collector(a-c) and where such particles or aggregates are stable at ambienttemperatures (or under refrigeration) and may be set aside for furtherprocessing. The B-stage particles can then be collected and molded intoa desired shape using standard plastics processing methods (e.g.,compression molding (d), extrusion (e), injection molding, etc.). Thisprocessing step provides further homogenization of the resultingcomposite since the B-stage particles retain thermoplastic propertiesfor a short period of time. Generally, this takes place at temperatureshigher than those typically used to solidify the sprayed droplets sothat the second curing agent is then activated, thereby ensuringcomplete and homogeneous crosslinking of the polymer matrix throughoutthe molded object.

In some embodiments, an “incipient wetting” technique is employed. Thisis a technique for depositing SWNTs on polymer or other surfaces fromdispersions in organic solvents (Barrera et al., International PatentApplication Serial No. PCT/US00/33291; and Barrera, JOM, 2000, 52, 38)and can be used to coat the surface of the B-stage particles withadditional SWNTs prior to forming the final composite. Deformation andmixing of the (temporarily) thermoplastic particles, as a result of theheat and mechanical shear at the start their final processing,distributes the additional SWNTs within the composite, thus increasingits SWNT content.

In a number of embodiments, solutions of SWNTs and one or more ofseveral epoxy prepolymers with a single curing agent are dispersed(dissolved) in an organic solvent (e.g., N,N-dimethyl formamide). Theseorganic liquid/prepolymer/SWNT systems can then be heated and quicklyatomized into fine droplets, which can then be sprayed into a preheatedchamber, and deposited onto a preheated hot surface. The process issuccessful in achieving solvent evaporation, polymerization of the epoxyresin, and integration of the solidifying droplets into coherent layersof composite, simultaneously, in the brief period between atomizationand droplet deposition. The success of such processes require that therapid rates of solvent evaporation and polymerization/cure beindependently controlled, so that the droplets solidify in flight beforethe SWNTs have a chance to separate from the emerging polymer andre-aggregate due to solvent depletion. However, when the solidifieddroplets reach their target surface, they must still retain residualreactivity in their nascent polymer, so as to coalesce and bond into acontinuous solid. If polymerization lags, the solvent evaporates leavingan insufficiently polymerized (still fluid) droplet, the SWNTs tend toexpel the organic molecules and re-clump (re-bundle). If polymerizationoutpaces evaporation, the droplets solidify too fast, tending to trapsolvent and form defective aggregates. In either of the latter twocases, the mechanical properties of the resulting composite are possiblychanged.

In some embodiments, solutions of liquid epoxy prepolymers are prepared(in organic solvents such as N,N-dimethylformamide) with one or moreamine curing agents, wherein such solutions typically comprise ca. 1weight % as-received SWNTs by weight. These solutions can then beatomized and sprayed onto a stationary surface, generating layeredspecimens of SWNTs/epoxy composites that show good SWNTs dispersion inthe epoxy matrix (see, e.g., FIG. 9). The products of such processeshave electrical properties, comparable to those of the pure epoxysystem, and they show an increase in electrical resistivity from ca.10⁺¹⁴ Ohm·m to ca. 10⁺⁰ Ohm·m (pure SWNTs: ca. 10⁻⁶ Ohm·m) as measuredby a four-point probe test. Corresponding increases in thermalconductivity are expected.

A number of functionalization (i.e., derivatization) methods (Mickelsonet al., Chem. Phys. Lett. 1998, 296, 188; Chen et al., J. Mater. Res.1998, 13, 2423; Boul et al., Chem. Phys Lett. 1999, 310, 367; Barr etal., J. Am. Chem. Soc. 2001, 123, 5348; Ying et al., Organic Letters2003, 5, 1471) have succeeded in covalently bonding several types oforganic groups to carbon nanotubes. These methods increase thesolubility of the resulting derivatized SWNTs in organic liquids (andthe diversity of solvents in which they can be dispersed/dissolved), andcan even provide covalent bonding between the SWNTs (or other CNT types)and the surrounding polymer matrix. Wherein the chemical treatments,necessary for these derivatizations do not disrupt the networkconformations of the nascent SWNTs and/or significantly degrade theirmechanical properties, their use in conjunction with the methods of thepresent invention should considerably enhance the range and versatilityof the resulting composites. These enhancements range from the use ofsolvents that are incompatible with nascent SWNTs, to the directincorporation of prepolymer systems into clumped nanotube networkswithout any solvent. One example is the use of “bucky paper” which hasbeen functionalized so that it is compatible with epoxy prepolymers.Stacks of the functionalized bucky paper are swollen with theprepolymer, and then cured in situ, using conventional moldingtechniques. The process will produce composites with very high SWNTcontent. Such composites have the potential of exhibiting mechanicalstrength and electrical conductivity, comparable to that of the pureSWNTs.

In some embodiments, to increase the dispersion of SWNTs, solutions ofliquid epoxy prepolymers are prepared (in organic solvents such asN,N-dimethylformamide) with one or more amine curing agents, wherein thesolution comprises ca. 0.1 weight % of carboxylic acid groupend-functionalized SWNTs. These can then be atomized and sprayed onto astationary surface, generating layered specimens of carboxylic acidfunctionalized SWNTs/epoxy composites that show increased SWNTsdispersion in the epoxy matrix (see FIGS. 11(A) and 11(B)) compared with0.1 weight % of as-received SWNTs/epoxy composites (see FIGS. 10(A) and10(B)).

In some embodiments, to increase the dispersion of SWNTs, solutions ofliquid epoxy prepolymers are prepared (in organic solvents such asN,N-dimethylformamide) with one or more amine curing agents, wherein thesolution comprises ca. 0.1 weight % of carboxylic acidsidewall-functionalized SWNTs. These can then be atomized and sprayedonto a stationary surface generating layered specimens of carboxylicacid sidewall functionalized SWNT/epoxy composites that show high SWNTsdispersion in the epoxy matrix (see FIGS. 12(A) and 12(B)) compared withothers (see FIGS. 10 and 11).

In some embodiments, the random orientation of CNTs can be observed byRaman spectroscopy with a polarized laser beam on the fracturedpolymer/CNT composites. Solutions of liquid epoxy prepolymers areprepared (typically in organic solvents such as N,N-dimethylformamide)with one or more amine curing agents, such solutions typicallycontaining ca. 0.1 weight % of as-received SWNTs. These can then beatomized and sprayed onto a stationary surface, generating layeredspecimens of the SWNT/epoxy composite. After irradiating the fracturedside of SWNT/epoxy composite with a polarized laser, there was nosignificant change in Raman intensity as a result of rotating thesample's position to the incident laser beam by: 0 degrees, 45 degrees,and 90 degrees (see FIG. 13, traces a-c). The SWNT orientation in thecomposites can be observed by the ratio of the radial breathing mode(RBM) peak and the tangential mode (G) peak of Raman spectroscopyrepresent. The resulting SWNT/epoxy composite showed no significantchange as a result of changing the incident laser beam angle. Therefore,SWNTs were randomly oriented in the composites (see FIG. 14).

In some embodiments, polymer/CNT can be aligned on a surface byadjusting process parameters, such as temperature of the preheatedsubstrate, the rate of the spray, concentration of theCNTs/prepolymer/organic liquid, etc. A single shot (spray) of 0.5 weight% as-received SWNTs/epoxy/DMF was sprayed onto a preheated substrate.Referring to FIGS. 15( a) and 15(b), when the produced SWNT/epoxycomposite sample was observed with optical microscopy, two differentimages were observed by changing the focal point with respect to sampleposition. By moving the focal point up and down, certain images are seento appear and disappear. From this observation, many palmtree-like-SWNTs/epoxy columns were vertically aligned on the substrate.A possible mechanism for the formation of this structure isschematically depicted in FIGS. 16( a)-16(i). Therefore, SWNT/epoxy canbe vertically aligned by this invention.

In some embodiments, epoxy systems utilizing dual curing agents areemployed. In such embodiments, the first agent (active at lowertemperature in the range of ca. 80-140° C.) effects rapid polymerizationof the resin to a B-stage (partially cured prepolymer) consistency,which stabilizes the penetration of the partially cured epoxy chainsinto the expanded SWNT networks. The second agent (active at hightemperature in the range of 10-200° C.) activates during subsequentprocessing of the B-staged particles, thus completing the cure(crosslinking) of the macromolecular network that has penetrated theexpanded, but still tangled, SWNTs. These two-stage systems can besprayed into a heated chamber and/or deposited onto a moving surface(such as a rotating disc). A cross-sectional view of a suitableapparatus for carrying out such processes is shown in FIG. 8. Note themultiple means of regulating spray volume and intensity, air and surfacetemperature, speed of rotation, and chamber pressure, in order toenhance control of the simultaneous solvent evaporation and cure toB-stage. Batches of the partially-cured prepolymer particles (with orwithout incipient wetting with additional SWNTs) can then be processedby high-shear extrusion, following the techniques of Shofner et al.(Shofner et al., J. Appl. Polym. Sci. 2003, 89, 3081; Shofner et al.,Composites: Part A 2003, 34; and M. L. Shofner: Ph. D. Thesis, RiceUniversity, 2004) in order to produce homogeneous SWNT/epoxy compositeswith the desired shape(s).

A process based on dissolution of SWNTs in super acids (Davis et al.,Macromolecules 2004, 37, 154) produces aligned SWNT ropes which are thenspun into fibers using various coagulants to precipitate out thenanotubes (wet spinning process). The diameter of the spun fibers isabout 3 orders of magnitude greater than the individual SWNTs. At thecurrent state of that invention, the integrity and strength of theresulting fibers is based on secondary chemical bonds formed between thealigned SWNT ropes. One way to increase fiber strength would be toincorporate the aligned SWNT ropes into interpenetrating networks ofcrosslinked and aligned high-strength polymer chains, such as aromaticpolyamides. The process would involve incorporation of the prepolymersin the SWNT solvent, their partial polymerization during the SWNTalignment process, and their further alignment and crosslinking as thefiber emerges from the spinnerets. A conceptual representation of theresulting fiber is shown in FIG. 17.

In conclusion, the present invention provides polymer/CNT composites,wherein the CNTs and polymer material form interpenetrating networks.Only by using the methods of the present invention can such compositesbe produced, wherein such composites possess property enhancement overthose composite systems not formed via interpenetrating networks.

All patents and publications referenced herein are hereby incorporatedby reference. It will be understood that certain of the above-describedstructures, functions, and operations of the above-described embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention as defined by the appendedclaims.

1. A B-stage powder comprising: a) CNTs; b) a polymer material thatforms interpenetrating networks with the CNTs, wherein the polymermaterial is at least partially cured to an extent so as to prevent are-bundling of the CNTs; and c) at least one un-activated curing agentcapable of further curing the polymeric material.
 2. The B-stage powderof claim 1, wherein the CNTs are SWNTs.
 3. The B-stage powder of claim1, wherein the CNTs are functionalized.
 4. The B-stage powder of claim2, wherein the SWNTs are functionalized.
 5. The B-stage powder of claim1, wherein the polymer material is selected from the group consisting ofepoxy resins, unsaturated polyester resins, vinyl ester resins, andcombinations thereof.
 6. The B-stage powder of claim 1, wherein theunactivated curing agent is selected from the group consisting ofaromatic amines, diaminodiphenyl sulfone, acid dianhydrides, borontrifluoride monoethylamine complexes, and combinations thereof.
 7. TheB-stage powder of claim 1, wherein the powder is comprised of particleshaving a size that ranges from about 1 micron to about 100 microns.
 8. Aprocess comprising the steps of: a) providing a plurality of B-stageparticles comprising partially cured polymer material; and b) furthercuring the polymer material to form bulk objects comprisinginterpenetrating networks of CNTs and polymeric material.
 9. The processof claim 8 further comprising a step of incipient-wetting the B-stageparticles with additional CNT material.
 10. The process of claim 8,further comprising a step of adding additional polymeric material to theB-stage particles.
 11. The process of claim 8, wherein the step offurther curing is done during a process selected from the groupconsisting of high-shear extrusion processes and injection molding. 12.The process of claim 8, wherein the step of further curing is done inconjunction with solid free-form fabrication.
 13. The process of claim8, wherein the step of further curing is done in conjunction with atleast one rapid prototyping technique.
 14. A polymer/CNT compositecomprising interpenetrating networks, wherein the composite is made by aprocess comprising the steps of: a) introducing CNTs and prepolymermolecules into a solvent to form a solvent mixture; b) atomizing thesolvent mixture into micro-droplets via spraying; c) rapidly removingthe solvent from the micro-droplets and, simultaneously, at leastpartially curing the prepolymer to provide solid polymer/CNT particles;and d) depositing the solid polymer/CNT particles on a surface to form apolymer/CNT composite layer.
 15. The polymer/CNT composite of claim 14,wherein the CNTs are SWNTs.
 16. The polymer/CNT composite of claim 14,wherein the CNTs are functionalized.
 17. The polymer/CNT composite ofclaim 14, wherein the prepolymer is selected from the group consistingof epoxy resins, unsaturated polyester resins, vinyl ester resins, andcombinations thereof.
 18. The polymer/CNT composite of claim 14, whereinthe prepolymer is at least partially cured using at least one curingagent selected from the group consisting of di-amines, poly-amines,imidazoles, and combinations thereof.
 19. The polymer/CNT composite ofclaim 14, wherein the step of introducing involves a technique selectedfrom the group consisting of sonication, mechanical shear, heating, andcombinations thereof.
 20. The polymer/CNT composite of claim 14, whereinthe step of rapidly removing solvent comprises exposure to a conditionselected from the group consisting of heating, mechanical pumping,evacuation, and combinations thereof.
 21. The polymer/CNT composite ofclaim 14, wherein the step of depositing the particles is done with heatto effect crosslinking.
 22. The polymer/CNT composite of claim 14,wherein the solid polymer/CNT particles have a size that ranges fromabout 1 micron to about 100 microns.
 23. The polymer/CNT composite ofclaim 14, wherein the CNTs are preferentially oriented within thecomposite.
 24. The polymer/CNT composite of claim 23, wherein thepreferential orientation is the result of extrusion, protrusion, orother processes used to orient fibrous reinforcement in polymers. 25.The polymer/CNT composite of claim 14, wherein CNT concentration withinthe polymer/CNT composite layer is graded in a particular direction. 26.The polymer/CNT composite of claim 14, wherein CNT concentration withinthe polymer/CNT composite layer is modulated with alternatingpolymer/CNT sublayers of differing CNT concentration.